The use of statins, a class of drugs that reduce plasma cholesterol levels in the management and prevention of coronary heart disease, has increased dramatically in the past decade and is likely to continue rising. There has been concern that statins may increase the risk of cancer, but other evidence points to a possible reduction in cancer risk. Our main goal is to evaluate the relationship between statin use and two site-specific cancers (breast and prostate) that have been shown to be responsive to statins in animal models, reported to increase or decrease among statin users in observational studies, and are of particular health burden in our community. Secondarily, we will assess the effect that duration of statin use and different types of statins have on the risk of breast and prostate cancers. This will be the first study to evaluate the association between statin use and stage at diagnosis for breast and prostate cancers, histology of breast cancer, and female reproductive/genital organ cancers. To meet our specific aims, we will conduct a retrospective cohort study from 1990 through 2003 within Group Health Cooperative (GHC), a large health maintenance organization in western Washington State. Cancer outcomes will be identified through the western Washington Surveillance, Epidemiology, and End Results registry. Information on statin use and covariates will be obtained from GHC automated data files, which include pharmacy records, laboratory values, hospitalizations, outpatient visits, and cause of death. Data from GHC's Breast Cancer Surveillance Program will be used to identify other covariates such as reproductive history, menopausal status, family history of cancer, body mass index, and frequency of mammography screening for the female cohort.